It has long been part of the leftist creed that the SAT and other standardized tests are biased against the culturally disadvantaged – racial minorities, the poor, etc. Those kids may be just as academically capable as more privileged kids, but the tests don’t show it.

But maybe SATs are biased against privileged kids. That’s the implication in a blog post by Greg Mankiw. Mankiw is not a liberal. In the Bush-Cheney first term, he was the head of the Council of Economic Advisors. He is also a Harvard professor and the author of a best-selling economics text book. Back in May he had a blog post called “A Regression I’d Like to See.” If tests are biased in the way liberals say they are, says Mankiw, let’s regress GPA on SAT scores and family income. The correlation with family income should be negative.

…a lower-income student should do better in college, holding reported SAT score constant, because he managed to get that SAT score without all those extra benefits.

In fact, the regression had been done, and Mankiw added this update:

Todd Stinebrickner, an economist at The University of Western Ontario, emails me this comment:

“Regardless, within the income groups we examine, students from higher income backgrounds have significantly higher grades throughout college conditional on college entrance exam . . . scores.” [Mankiw added the boldface]

What this means is that if you are a college admissions officer trying to identify the students who will do best in college, as measured by grades, you would give positive rather than negative weight on family income.

Not to give positive weight to income, therefore, is bias against those with higher incomes.

To see what Mankiw means, look at some made-up data on two groups. To keep things civil, I’m just going to call them Group One and Group Two. (You might imagine them as White and Black, Richer and Poorer, or whatever your preferred categories of injustice are. I’m sticking with One and Two.) Following Mankiw, we regress GPA on SAT scores. That is, we use SAT scores as our predictor and we measure how well they predict students’ performance in college (their GPA).

In both groups, the higher the SAT, the higher the GPA. As the regression line shows, the test is a good predictor of performance. But you can also see that the Group One students are higher on both. If we put the two groups together we get this.

Just as Mankiw says, if you’re a college admissions director and you want the students who do best, at any level of SAT score, you should give preference to Group One. For example, look at all the students who scored 500 on the SAT (i.e., holding SAT constant at 500). The Group One kids got better grades than did the Group Two kids. So just using the SATs, without taking the Group factor (e..g., income ) into account, biases things against Group One. The Group One students can complain: “the SAT underestimates our abilities, so the SAT is biased against us.”

Case closed? Not yet. I hesitate to go up against an academic superstar like Mankiw, and I don’t want to insult him (I’ll leave that to Paul Krugman). But there are two ways to regress the data. So there’s another regression, maybe one that Mankiw does not want to see.

What happens if we take the same data and regress SAT scores on GPA? Now GPA is our predictor variable. In effect, we’re using it as an indicator of how smart the student really is, the same way we used the SAT in the first graph.

Let’s hold GPA constant at 3.0. The Group One students at that GPA have, on average, higher SAT scores. So the Group Two students can legitimately say, “We’re just as smart as the Group One kids; we have the same GPA. But the SAT gives the impression that we’re less smart. So the SAT is biased against us.”

So where are we?

The test makers say that it’s a good test – it predicts who will do well in college.

The Group One students say the test is biased against them.

The Group Two students say the test is biased against them.

And they all are right.

————————

Huge hat tip to my brother, S.A. Livingston. He told me of this idea (it dates back to a paper from the1970s by Nancy Cole) and provided the made-up data to illustrate it. He also suggested these lines from Gilbert and Sullivan:

And you’ll allow, as I expect
That they are right to so object
And I am right, and you are right
And everything is quite correct.

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